1. Field of the Invention
The present invention relates to flexible energy control sheets and assemblies which include at least one optical, thin film, coating layer for spectral control and which have enhanced water vapor permeability.
2. Background Information
For the past few decades, flexible solar control sheets have been in use to improve the energy transmission, and appearance of transparent glazing used in commercial buildings, residential buildings and vehicles. This invention relates to solar control sheets that are retro-fitted to already installed transparent glazing surfaces, or laminated to a glass or other glazing material surface as part of the original assembly of a glazing product. In this second case, a solar control sheet is installed on a transparent glazing surface as part of a fenestration manufacturing process before the window is installed in a building or vehicle.
The purpose for using these flexible energy control sheets is to alter the solar energy transmission, reflection, absorption, and emission of glazing. The most common function is to reduce solar heat gain thereby improving comfort and reducing cooling load within an architectural or transportation structure. Some energy control sheets are designed so that the surface of the sheet facing away from the rigid glazing to which the sheet is attached has high thermal infrared reflectivity. Such low emissivity sheets reduce thermal energy loss through glazing and contribute to reduction of heating energy requirements when outdoor temperatures are below indoor temperatures in a building or vehicle. Alteration of the visible, and infrared spectral characteristics of glazing is primarily done with optical thin film coatings. Although many alternative designs exist, the most commonly used optical thin film structures in solar control sheets may be categorized into three basic types. The simplest sheets reduce light transmission evenly in the visible and infrared wavelengths. These sheets are not considered spectrally selective, and usually contain one thin film layer consisting of an optically neutral nickel alloy. The second type of solar control sheet uses an infrared reflecting metal such as aluminum, copper or silver as its thin film layer and the reflection level in the infrared wavelengths is increased in these sheets making them somewhat spectrally selective. The third type of solar control sheet also contains infrared reflecting metals but makes use of thin optical interference layers as well. The optical interference layers are usually nonabsorbing or slightly absorbing dielectric layers. The interference layers antireflect the metals and result in solar control sheets with high visible transmission, high infrared reflectance, low visible reflectance, and low infrared transmission. Some combinations of infrared reflecting metals and interference layers result in sheets with high spectral selectivity. This invention concerns flexible solar control sheets which contain such thin film layers and are attached to a surface of rigid transparent glazing by an adhesive.
For purposes of optical clarity, solar control sheets must be attached to the surface of a rigid transparent glazing material such as glass with no trapped air or other sources of optical distortion. Except in certain manufacturing environments where a sheet can be rolled onto a glass surface with precision equipment, dry adhesive cannot be attached to dry glass without incorporating air. Particularly when applied to already installed glazing, proper positioning of a solar control sheet on a glazing surface is a problem due to the tendency for instantaneous sticking of adhesive to glass. The two problems of air entrapment and positioning are solved by the addition of water or a dilute solution of water and surfactant between the solar control sheet's adhesive and the glass surface. With the presence of this aqueous solution acting as a lubricant and spacer, positioning of the sheet and subsequent squeezing out of trapped air is relatively easy. Once all positioning and air removal is complete, squeezing or squeegeeing of residual solution out from between solar control sheet and glass is done until as much liquid is removed by this technique as possible. Complete removal of water and surfactant is not possible by this squeezing process. Some residual solution always remains between the solar control sheet and the rigid glazing surface.
The result of this water aided attachment process and its leftover layer of aqueous solution is not without negative consequences to the solar control sheet. The water portion of the remaining solution diffuses into the materials from which the solar control sheet is assembled and, if it remains too long in this assembly, causes undesirable chemical changes. The most immediate change is the formation of a two phase mixture of water and adhesive polymer in the adhesive layers that are part of the solar control sheet assembly. This two phase mixture causes the scattering of visible light which gives the sheet a milky translucent appearance. Solar control sheets applied to glazing and that retain water for more than approximately six hours will form the mixtures of adhesive and water. Although not permanent, the initial formation and duration of this milky appearance is dependent on the overall water vapor permeability of the solar control sheet. If water diffuses through and evaporates from the solar control sheet within a few hours and before the mixture of adhesive and water has time to form, the milky appearance will not occur. Solar control sheets that dry within this few hour period under ambient conditions that are not unusually cold and humid (&lt;5.degree. C. and &gt;70% relative humidity), are hereafter referred to as "rapid drying" or "water permeable".
Other problems, as a consequence of the residual, aqueous, surfactant solution, are corrosion of metal thin film layers within the solar control sheet assembly and optical distortion of the adhesive layer in contact with the rigid glazing surface. Corrosion of the metal layers occurs from the water contained within the assembly acting as an electrolyte and causing galvanic chemical activity. The metal layers present for spectral control are less than 100 nanometers thick and even small amounts of galvanic chemical activity will destroy their intended optical function. Optical distortion within the attachment adhesive layer is cause by coalescence of the residual solution into pools between the adhesive and rigid glazing. These pools appear exactly like water filled blisters and as long as they are present they distort the optical clarity of the solar control sheet. Often these blisters, if present for days without drying out, will create permanent deformities in the adhesive and subsequent undesirable permanent optical distortion in the solar control sheet. The occurrence of both corrosion of the metal layers and adhesive distortion are related to the overall water vapor permeability of the solar control sheet.
Water vapor permeability of the various components of the solar control sheet assembly are not equal. The polymer layers within this assembly which typically include polyethylene terephthalate sheets or other polymer sheets, adhesive layers, and hard polymer layers for abrasion resistance all have sufficient water vapor permeability to avoid the previously mentioned problems associated with residual attachment solution. If these were the only layers present in the solar control sheet, residual solution would diffuse through the film rapidly enough to cause no detrimental effects. Sufficiently rapid diffusion of water through and evaporation away from the sheet until water content equilibrium is reached with the ambient humidity, hereafter known as "drying", needs to occur within a few hours for none of the problems to occur, especially for the milky appearance.
The optical thin film layers deposited upon the polymer sheets within the solar control sheet assembly, minus some exceptions to be noted, do not share the same degree of water vapor permeability as the polymer layers. Most vacuum deposited thin film layers typically used for spectral control are excellent barriers to diffusion of water vapor. These thin film layers are the water vapor diffusion rate determining segment of a solar control sheet and it is the permeability of these layers that is the focus of this invention.
Solar control sheets that are water vapor permeable and therefore non-clouding, non-blistering and less prone to corrosion are more commercially viable than those that are water impermeable. A large part of the market for solar control sheets is application to automotive glazing. For safety reasons and to achieve a pleasing appearance, it is important for solar control sheets applied to automotive glazing to exhibit no clouding or distortion when they are applied. Water permeability related problems causing poor optical clarity of a solar control sheet in the automotive market will severely limit its salability.
For solar control sheets applied to the architectural glazing market, the water permeability related problems are not a safety hazard as in the automotive market but will still substantially limit the salability. Distortion and cloudiness lasting more than a day or two are generally considered unacceptable in a solar control sheet applied to residential or commercial buildings. When solar control sheets are applied to glazing as part of a window manufacturing process, they are usually applied to the one of the internal surfaces of a dual pane insulating glass unit. Inclusion of water vapor within the internal gas space in an insulating glass unit will cause condensation formation for the life of the unit which is considered unacceptable in the industry. Therefore, before the two glass panes and the gas space are sealed, the solar control sheet must be thoroughly dried. Drying an impermeable solar control sheet is expensive due to the time required and would not be salable to this manufacturing market.
As noted above, solar control sheets which have sufficient water permeability for rapid drying are highly desirable in the technological field to which this invention applies. Also noted is the fact that of all the layers within the solar control sheet assembly, the vacuum deposited thin film layers are the water permeability limiting portion. It is the physical structure of thin film layers that is the primary determinant of the rate at which water may pass through a solar control sheet. Films formed of tightly compacted atoms or molecules are barriers to water passage. Films that have columnar or crystalline structures and having open spaces between columns or crystals are more permeable to water. The physical structure of these thin film layers varies depending on their deposition method, the materials from which they are formed and their thickness.
Vacuum deposition methods by which typical thin film solar control coatings are made are either thermal evaporation or direct current magnetron sputtering. These deposition processes are distinguished by how the individual atoms or molecules are separated from the source material and accelerated towards the substrate. In the process of thermal evaporation, the source material is heated until atoms or molecules leave its surface as vapor which recondenses on the substrate. In the sputtering process, kinetic energy of a positively charged ion accelerated towards the negatively biased source material (the sputtering target) transfers its energy to the atoms or molecules at the surface of the source material. This transfer of energy results in the chipping off of surface atoms or molecules. An important difference in these two deposition processes is the kinetic energy they impart to the depositing atoms and molecules. Atoms and molecules deposited with thermal evaporation carry low levels of kinetic energy (less than 1 electron volt) and are more likely to form thin films with open structures that are water permeable. Solar control sheets containing transparent layers of thermally evaporated aluminum or nickel are common and sufficiently water permeable to be considered rapid drying products.
The vacuum deposition method of direct current magnetron sputtering is the most commonly used process for forming thin film coatings of metals other than aluminum for solar control sheets and is characterized by the higher level of kinetic energy it imparts to the depositing atoms (1 to 10 electron volts). Depositing metal atoms carrying high kinetic energies are far more likely to form thin film layers with tight compact structures and are usually insufficiently water permeable to be considered fast drying. Neither transparent nickel or aluminum thin films are considered rapid drying in a solar control sheet when deposited by sputtering. The metals of gold, copper and silver and their alloys which have high infrared reflectivity and are necessary to produce spectrally selective, high visible transmission, low infrared transmission solar control sheets are typically deposited by direct current magnetron sputtering. They generally do not form water permeable films.
The commonly manufactured solar control sheets with neutral gray, nonspectrally selective optical characteristics have relatively similar reflectance and transmission characteristics across all of the ultraviolet, visible and infrared spectrum. The thin film metals used to produce the nonspectrally selective solar control sheets are typically titanium, chromium, iron, nickel, niobium, molybdenum, and alloys of these. The most common method of depositing these thin film metals for solar control sheets is direct current magnetron sputtering, and when deposited by sputtering, they generally are not water permeable.
In some solar control sheet products, it is desirable to achieve greater spectral selectivity than that exhibited by the neutral grey metal layers described above. Solar control sheets with visible transmission greater than 50%, visible reflectance less than 15%, infrared reflectance greater than 50%, and infrared transmission less than 15% are useful in the marketplace due to their potential for improving the energy efficiency of architectural or automotive windows. The spectral selectivity is typically achieved by alternating thin film layers consisting of infrared reflecting metals and dielectric optical interference layers. The metal layers usually consist of direct current magnetron sputtered silver, copper, gold or their alloys. The dielectric layers may be sputtered or thermally evaporated, and their effect is designed to reduce reflectance and raise transmission of the metals in the visible wavelengths. These multilayer spectrally selective films are particularly impermeable to water.
Thermal evaporation can be accomplished by a few different methods. For nonmetallic materials with low vapor pressures such as titanium dioxide, electron beam evaporation is sometimes used. In the electron beam process, a beam of electrons is aimed directly at the evaporant held in a high temperature crucible. Temperatures high enough to evaporate virtually any material can be reached by this technique. When oxides or other compounds are evaporated, the atoms in the molecular structure of the compound are often dissociated due to the extreme heat. For example, when TiO.sub.2 is brought up to its vacuum vaporization temperature, some of the titanium and oxygen atoms separate and a portion of the released oxygen is pumped away by the vacuum pumps. Consequently, the coating is not stoichiometric TiO.sub.2 but is instead a titanium rich compound which is optically absorbing. Vacuum coaters typically require clear (non-absorbing) TiO.sub.2 and compensate for the lost gas by adding extra oxygen into the chamber during the coating process. However, in other technological fields which provide a metal oxide coating, such as the technological field relating to the manufacture of heat mirrors on lenses, glass and small polymer substrates, it is important to carefully control the amount of extra oxygen added during the coating process because if too much oxygen is added, the deposited titanium dioxide becomes porous. Porosity in these other technological fields is undesirable and precautions are used to prevent its occurrence so as to keep the deposition clear but not porous.
Such porous coatings in these other technological fields have never been associated with any method of making a porous metal coating on a substrate especially a thin film substrate for retrofit application on windows and consequently no one has ever used such coatings to produce a porosity inducing surface for inducing porosity of materials, such as metals, which are coated on solar control sheets. This is not surprising in view of the undesirability of porosity in these other technological fields.
As noted above, it is known in other technological fields to make heat mirrors on lenses, glass and small polymer substrates which include a thin film structure having the layers TiO.sub.2 /Ag/TiO.sub.2 wherein TiO.sub.2 serves as a dielectric layer. Many other dielectrics are used also for heat mirrors on lenses, glass, and small polymer substrates. Other typically used dielectrics include ZnO, ZnS, Nb.sub.2 O.sub.5, SnO.sub.2, Ta.sub.2 O.sub.5, or In.sub.2 O.sub.3. Occasionally the process produces poor quality porous stacks. However, practitioners in these other technological fields never attributed such porosity with any specific characteristics of a layer upon which another material is deposited. Thus the undesirable porosity of such a stack could be attributed to other factors, such as the thickness of the layers or coating conditions for the metal layer rather than to the selection of physical characteristics of a layer upon which other layers are deposited. In any event, practitioners in these other technological fields never regulated the porosity of a primer layer in order to achieve porosity in a subsequently applied metal layer nor would they find it desirable to do so. Thus, it was never apparent to such practitioners that a porous metal coating could be obtained through the use of a process which employs a metal coating step conducted under conditions which would normally produce a non-porous coating but for the selection of a primer layer having certain porosity inducing characteristics.
Resistance evaporation is another commonly used evaporation technique to provide a coating on an article. This process is similar to electron beam evaporation except electric current flowing through a heating element is used as the heat source to evaporate the coating material. This process also requires the addition of extra oxygen to the chamber for the same reasons noted above with respect to the electron beam evaporation coating. Resistive evaporation techniques as used in other technological fields typically require careful control of the oxygen content to keep the deposition clear but not porous. In other technological fields, e-beam and resistive evaporation are used for thousands of different types of coatings. Most of these coatings require careful control of oxygen to minimize porosity and absorption. Consequently, the same statements discussed above regarding electron beam evaporative coating are applicable to resistance evaporative coating techniques.
In other technological fields, porosity is considered detrimental. For example, it is known in the glass industry that a reactively sputtered zinc oxide coating can be made by sputtering a zinc target in a sputtering gas (e.g. argon) containing sufficient oxygen so that the entire surface of the zinc target is converted to the oxidized state. Under these conditions the properties of the deposited coating changes dramatically since the active portion of the sputtering target, its surface, is no longer metallic but is the metal oxide. For most sputtered materials, as the target surface goes from metal to oxide, the coating also goes from metal to oxide and usually both are relatively impermeable to water vapor. For some materials, as the chemical transition from metal to oxide occurs, so does the structural transition from non-porous (impermeable to water vapor) to porous. Materials which are known to follow this pattern are zirconium and zinc.
Metal oxides or other compounds which may be used in solar control sheets are more complex with regard to thin film structure and water permeability characteristics. Metal oxides and other chemical compound thin films generally follow the same rule of permeable structure as metals; that is the higher the kinetic energy of the atoms the tighter the film structure. For metal oxides and other compounds, however, there are other factors which can dictate whether the thin film structures are water permeable or impermeable. The most important of these factors is the makeup and pressure of the background gas as the films are deposited in the vacuum chamber. High background pressures of reactive gas, particularly amounts in excess of that which is required to produce a stoichiometric compound will result in thin films with open permeable structures. In fact, one embodiment of this invention makes use of excess amounts of reactive gases such as oxygen to produce thin films with specific degrees of openness in their structure.
Table 1 compares the water permeability of optically transparent, thin films deposited on a polyethylene terephthalate sheet substrate.
TABLE 1 ______________________________________ Water Permeable Thin Films Water Impermeable Thin Films ______________________________________ evaporated aluminum sputtered aluminum evaporated nickel sputtered nickel sputtered silver evaporated oxides sputtered copper some oxides sputtered in excess oxygen most reactively sputtered oxides without excess oxygen nickel alloys sputtered in argon pressures between 40 and 60 microns of mercury ______________________________________
The water permeability of solar control sheets is measured in terms of a water vapor transmission rate (WVTR). The units of measurement are usually grams/square meter/day. Generally, if the WVTR of a solar control sheet is 2 grams/square meter/day or more, drying time is sufficiently short as to avoid cloudiness caused by entrapment of water between the film and the glass. Therefore, solar control sheets which have a WVTR of at least 2 grams/square meter/day are considered as being sufficiently water permeable to be considered rapid drying in this invention. Those which have a WVTR less than 2 grams/square meter/day are considered as being water impermeable in this invention and are not considered fast drying. A typical solar control sheet assembly as shown in FIG. 1 consisting of a glazing attachment adhesive, a 12 micron thick UV absorbing polyester (polyethylene terephthalate) sheet, a laminating adhesive, a 25 micron polyester sheet, and a 1 micron acrylic abrasion resistant coating but no thin film layers to limit water permeability, have a WVTR of approximately 20 grams/square meter/day. The same solar control sheet assembly described above but containing vacuum deposited thin film layers may have a WVTR from 0.1 to 20 grams/square meter/day. The thin film layers in solar control sheet assemblies are the component that determines whether a sheet is to be rapid drying or not.
Conventional devices known to those skilled in the art may be used to measure the water permeability of a solar control sheet. One such device used to measure permeability, commonly referred to as a "mocon", is commercially available from Modern Controls Inc., 6820 Shingle Creek Parkway, Minneapolis, Minn. 55430. The water vapor transmission rates described in relation to this invention are obtained by operating the instrument in accordance with ASTM test procedure F372-73 (reapproved 1984).sup..delta.2. The instrument uses a removable diffusion cell having upper and lower halves. In operation, test samples of a sheet are cut into pieces about 4 inches by 4 inches (10 cm.times.10 cm). The sample sheet is mounted between the upper and lower halves of a removable diffusion cell so as to form a divider between two enclosed chambers (upper and lower halves). The lower volume of the assembled cell block contains pads moistened with distilled water or saturated salt solution (NaCl). The upper volume is vented through two openings that permit a constant flow of dry air to pass across one side of the film. The mating internal surfaces of the diffusion cell define an area of 50 square centimeters.
The test sheet mounted in the diffusion cell is clamped into the test chamber. When inserted, the sheet is exposed to a continuous flow of dry air across the upper side while the bottom side is exposed to water vapor from the moistened pads in the humid cavity. Gas leaving the dry cavity via the exhaust port consist of a mixture of air and water vapor in a ratio determined by the dry air purge rate and the rate of moisture transmission through the sheet barrier. Thus, it can be easily understood that if the flow rate of dry air into the cell is maintained at some arbitrary constant value, the resulting water vapor density in the exhaust line will be determined by the sheet water vapor transmission rate.
The water vapor concentration of the diffusion cell exhaust flow is monitored by an infrared detector. Over the concentration range of interest, the detector output displayed by the digital meter is a linear function of the transmission rate of moisture through the sheet.
Three approaches are available for overcoming the above noted clouding problem of solar control sheets in this technological field. In one approach, the use of water in the mounting process could be eliminated. However, the water serves to promote slip during the mounting process so that the film can be readily positioned on the window, and the water serves to facilitate the removal of trapped air bubbles. Thus, use of the water to solve the clouding problem will create extreme application difficulties. At this time there is no known way to apply solar control sheets to existing glazing without the use of an aqueous solution.
Another approach could involve using non-clouding adhesives. This technology is presently practiced in the industry by some participants to reduce the clouding problem. However, this does not solve the other problems of corrosion of the metal thin film layers and optical distortion associated with long term entrapment of water. Thus, elimination of the non-clouding adhesives would not solve all the problems associated with the use of water as a mounting media.
Another approach involves substituting impermeable, thin film, optical layers with permeable layers which have the requisite permeability so that the water can rapidly diffuse through the film. As noted above, the number of permeable thin film layers is limited. Solar control sheets that are the most spectrally selective and exhibiting the most desirable optical properties generally make use of silver, copper, or gold metal layers. These metals cannot be made permeable through standard deposition techniques practiced today.